Cable telecommunications systems, often referred to as broadband communication systems (BCSs), have been known for a number of years and are currently gaining in popularity and coverage for the distribution of television programming, telephone service and networking of computers such as providing Internet access since they can carry many signals over a wide bandwidth with little, if any, interference or significant distortion, particularly as data transmission rates have increased to accommodate ultra high definition television, increased volume of digital communication and the like. By the same token, since these communications are intended to be confined within the cable system, the increased bandwidth required for such communications need not be allocated from the available bandwidth for other communications such as radio, navigation, GPS, emergency or navigation communications and the like which must be transmitted as electromagnetic waves through the environment. However, flaws in cable shielding in cable telecommunication systems can allow signal egress which can potentially interfere with broadcast communications and potentially cause hazards. Reciprocally, flaws in cable shielding can permit signal ingress into the cable from the environment and degrade or interfere with the signals being carried by the cable telecommunication system. Therefore, such flaws must be quickly discovered and remedied as they occur due to weather, mechanical damage, aging or the like.
Detection of cable shielding flaws in the distribution portion of a Broadband Communication System (BCS) is generally achieved through detection of the signal carried by the cable transmission system that has leaked into the environment, essentially by being broadcast from the shielding flaw. Detection of a signal that has leaked or egressed from a cable flaw is generally performed in two stages: first, by a receiver in a mobile vehicle driven in the general vicinity of installed cables that associates a received signal with a location of the mobile vehicle using a global positioning system (GPS) receiver which thus reports a general location of a shielding flaw and, second, by a hand-held instrument that can allow repair personnel to follow increasing signal strength to the exact location of the shielding flaw so that repairs and/or maintenance can be carried out.
A similar procedure but on a smaller scale may be carried out during installation or repair of equipment at a subscriber location to qualify the installation and equipment as being capable of providing high quality service to the subscriber. However, it is more common to disconnect the subscriber installation from the BCS and to detect any signal increasing into the subscriber installation from the environment. If such an ingress signal is detected, the process must be repeated at different locations in the subscriber installation in order to determine the location(s) of the shielding flaw(s). Further, although detection of ingress can also be helpful in detecting sources of noise that originate within the subscriber's premises and immediate environment so that remedial action may be taken in regard to the noise source, the detection of ingress signals is dependent on the level and frequencies of ambient electrical noise and broadcast or over-the-air signals at the subscriber location to determine the quality of shielding integrity in the subscriber location. Therefore, if the ambient electrical signal/noise levels are low at a subscriber location, significant egress may still occur even though no signal ingress is detected. Therefore, techniques for detecting signal egress as a measure of susceptibility or resistance to signal or noise ingress within a subscriber location is more rigorous and quantitative than direct measurement of ingress.
Of course, such signal egress detection must be carried out in an environment in which noise as well as broadcast signals will also be present in the same frequency bands. Accordingly, a problem with all such systems is to identify a received signal as one originating in the cable telecommunications system and numerous techniques have been developed to effectively verify or authenticate a detected signal as an egress signal. An additional issue that follows from this problem is that a signal which is unique to the cable telecommunication system and distinguishable from broadband noise (e.g. a marker signal) has the potential for interfering with the signal carried by the cable telecommunication system and/or necessarily consumes a finite amount of bandwidth if within the frequency band of the payload signals on the BCS and of more than minimal amplitude.
An exemplary system seeking to provide a solution to these related issues is disclosed in U.S. Pat. No. 4,072,899, issued Feb. 7, 1978, to Richard L. Shimp, which is hereby fully incorporated by reference. In the system disclosed therein, a variable frequency (e.g. “warbled”) audio tone is transmitted over the BCS, general by being added as a marker signal to the signal carried by the telecommunication system. Such an audio signal can be easily detected by a narrow band portable receiver such that the audio tone can be perceived and followed by maintenance personnel while being easily filtered from or having little effect on the other upstream or downstream signals carried by the cable telecommunication system. This arrangement has proven highly successful even though cable shielding flaws may be frequency selective; allowing egress at some frequencies but not others.
However, at the present time, the need to carry ever greater amounts of information (e.g. for high definition television (HDTV) and the like) has resulted in the choice of complex modulation schemes such as quadrature amplitude modulation (QAM) to multiplex signals which are, themselves, more complex and have increased data content. In general, a plurality of QAM multiplexers (often referred to as QAM generators) are used, each carrying a small number of channels of information, and their outputs are combined by allocating contiguous spectral bands to respective QAM multiplexers. The output of a QAM multiplexer or a plurality thereof is often statistically indistinguishable from ambient noise in the environment in which detection must be performed.
Accordingly, currently known techniques of signal egress detection have required the allocation of significant bandwidth (e.g. the equivalent of a band corresponding to a QAM multiplexer or at least the bandwidth corresponding to a television program channel) in order to provide a sufficiently complex signal for detection and identification without causing interference with other information carried by the cable telecommunication system. Allocation of such bandwidth has also been essential to measurement of the strength of signal egress allowing repairs to be prioritized and to assure compliance with regulations governing the operation of cable telecommunication systems.
Such an allocation of bandwidth thus reduces the otherwise available bandwidth of the cable telecommunication system and is essentially a large fixed cost of operating the system. Even with the allocation of economically significant bandwidth to the shielding flaw detection function, detection is not robust due to the limitation of marker signal power transmitted through a BCS in order to avoid interference and, where two or more cable telecommunication systems may be present in the same geographic area, identification of the system having the shielding flaw can often not be performed unless the marker signal is particularly complex; requiring more than minimal bandwidth allocation.
Additionally, a cable shielding flaw may only permit leakage of a power level that is too low to be detected amid ambient noise but is, nevertheless, an imperfection in shielding integrity which may be a point from which the leakage signal may increase to significant levels over time and/or may become a point of susceptibility for allowing extraneous electromagnetic energy to penetrate the flaw and interfere with payload signaling.